Press brakes commonly are equipped with a lower table and an upper table, one of which, commonly the lower table, is vertically movable toward the other table. Forming tools are mounted to the tables so that when the tables are brought together, a workpiece between the forming tables is bent into an appropriate shape. It is common for the upper table to include a male forming die having a bottom workpiece-deforming surface of a desired shape, such as a right angled bend, and for the bottom table to have an appropriately shaped and aligned die so that when the die is brought together with the forming tool, a workpiece between the two is pressed by the forming tool into the die and thus is given an appropriate bent shape. The forming tools and dies commonly are horizontally elongated so that workpieces of various widths can be accommodated.
It often is necessary to exchange forming tools and dies when a different bending operation is to be performed. The dies, commonly resting on the bottom table of a press brake, are readily removed and exchanged for others. The forming tools that usually are mounted to the upper table of a press brake often are not easily replaced. Forming tools usually are held by a C clamp or other holder to the horizontally elongated bed of the upper table. Once the clamp has been loosened, the forming tool can, in some instances, be removed downwardly, and in others, must be removed by horizontally sliding it from the clamp. If a long forming tool is to be replaced, it becomes difficult to slide the forming tool from its clamp because of the proximity of neighboring clamps and forming tools which may themselves have to be removed in order to complete the tool exchange process.
Because long forming tools can be quite heavy, when a clamp is loosened to the point that the tool can be removed by moving it downwardly, a tool may slip and fall, causing potential injury to press brake operators.
Several press brake holders have been devised in an effort to facilitate the exchange of one forming tool for another. For example, Treillet, U.S. Pat. No. 4,993,255 discloses a tool holder that is attached by means of a C clamp to the bed of the upper table. Through the use of a camming mechanism, the upwardly extending shank of a forming tool is captured between a pivotable clamp and a portion of the holder, the shank and clamp having cooperating surfaces enabling the tool to be readily inserted in the holder. A locking cam is employed to lock the clamp against the forming tool. Kawano, U.S. Pat. No. 5,513,514, U.S. Pat. No. 5,511,407, U.S. Pat. No. 5,572,902, and European patent publication 0 644 002 A2 show tool holders of the same general type in which a pivoting clamp is employed to receive the shank of a tool between it and the mounting plate of the holder. In each of these patents, the holder is equipped with a threaded mechanism operated by a lever that pivots from side to side to lock and unlock the clamp, force being transmitted from the lever to the clamp via a spring structure.
It remains difficult to remove a forming tool from a press brake holder, for the reasons stressed above. Care must be taken to avoid dropping press brake tooling as tools are exchanged in a press brake holder, and care must also be taken during attachment of a tool holder to the upper table of a press brake inasmuch as loosening of the clamps currently employed to hold a press brake holder to the bed may permit the holder to fall of its own weight. Moreover, with certain tool holders for forming tools, the force which the clamp exerts against the holder to maintain it in position varies along the length of the clamp; this, in turn, may cause unintentional movement of the forming tool with respect to the tool holder.